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Bonsai and extreme temperatures – Heat

With the recent heat waves that we’ve had in Perth (*recent upon posting), I thought it might be an idea to touch on the subject of extreme heat, and how it can affect our bonsai, and more importantly, what we can do to keep them cool.

So you all know that I am still learning the art, but I guess I’ll be saying that for many years to come, but one of the first things that I learnt with my bonsai, was how to take care of them when the temperatures start to soar. The weekend just passed was a record breaker here in Perth, with temps in my area hitting 48/49 degrees celsius. When it gets this hot, you need to take precautions, otherwise your bonsai will simply not survive. These are some tips that I have compiled, and use myself.

Tip#1: Probably the most obvious suggestion, would be to water your plants more. Sprinkler or automatic watering systems work quite well in this situation, however I would not reccomend relying on them, as automated systems can break or block, so just be wary. Watering your plants will obviously cool them down to a certain extent, but you need to be aware that in extreme temps, the water can quickly heat up, and may ‘cook’ the roots. A black pot sitting in direct sunlight will heat up VERY quickly for example. It shouldn’t be a major concern for most, as it would need to be quite a high temp to get to that point. I tend to move my watering up to two or three times daily during the hot periods, but you really need to watch your plants and the soil, as they will tell you when they need the water.

Tip#2: Another obvious one, would be to move your plants to a shady area for the duration of the heatwave. This would probably be the simplest option, as well as the most effective. Keeping your bonsai in a shady area will prevent the water evaporating quickly, and will also keep them out of direct sunlight, which can heat up a pot very quickly (and add to the evaporation rate). The best shading method would be to cover your bonsai with 30-50% shade cloth, but failing that, moving them to a shady area under a tree would be fine. I will sometimes move mine so that they only receive morning light, but are shaded for the afternoon which is when the heat is at its worst.

Tip#3: Some simple mulch or pebbles/stones covering the top of the bonsai. This will also slow the evaporation rate of the water, meaning the bonsai will stay cooler for longer. I personally use gravel to cover the tops of my bonsai, but I have heard of Lucrene Hay being used, as well as many other different types of pebbles or rocks. Some forms of mulch may actually hinder you, as it can prevent you seeing the soil and knowing whether your bonsai is dry or not, so just pay a bit more attention to this if using a mulch like hay.

Tip#4: A very good way to keep them cool, is by sitting you bonsai in a gravel tray filled with water. I fill the tray with gravel, then fill the tray with water and sit my bonsai on top of the gravel, making sure that there is a gap between the water level and the bottom of the pot. The idea is that as the water evaporates, it is drawn up through the pot, keeping it cooler for longer. This is pretty much a nessecity if you are growing Shohin or smaller bonsai, as they will dry up in the blink of an eye in the midday heat.

Tip#5: Would be to use wet carpet to lay over your soil. Make sure it is cut to the correct shape of your pot, with a hole in the middle, and cut going from the hole to the edge (that will allow you to wrap it around the trunk). That way, you can lay the carpet over the soil of your bonsai (so the trunk is coming through the hole), which will allow for the soil to stay damp for much longer. I have also heard of newspaper being used to this affect. While it may not look the best, it is very effective in keeping the plant cool, and the keeping the evaporation rate down.

Tip#6: Another thing to be aware of, and something that suprised me, is that the bigger the pot, the faster it will dry out. So for example, a large cascade pot will dry out much faster then a regular bonsai pot. This is due to the surface area that is exposed to the sun. The more surface area on a pot, the more heat it will absorb, and a hot pot is not a good pot. So just watch your big pots as much as you do you the small, as they will dry out quickly too. This can also be said for any type of rock plantings, like ROR (root over rock), planted in rock, or slab plantings. The rock not only gives more surface area to heat up, it also stores the heat, and continues leach and evaporate any moisture in your soil long after the temperatures drop.

These are just a few tips that I have learnt (sometimes the hard way) that will hopefully help keep your bonsai alive during the extremely hot days that come around during summer.

Please feel free to post your own tips in the comments, as this post will be updated as I discover more.